Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous inkjet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles. These small particles can be stabilized against flocculation and settling by the use of distinct dispersing agents such as surfactants, oligomers, or polymers, or they can be directly functionalized to provide a self-dispersing characteristic. In either case the carrier medium can be a liquid or a solid at room temperature. Commonly used carrier media include water, mixtures of water and organic co-solvents, and high boiling organic solvents such as hydrocarbons, esters, ketones, alcohols, and ethers.
Pigment-based inkjet inks are often preferred over dye-based inkjet inks because of the superior image stability typically observed with the pigment-based inks. Self-dispersed pigments in turn are often preferred over surfactant-dispersed, oligomer-dispersed or polymer-dispersed pigments because of their greater stability to a variety of ink formulations and environmental keeping conditions. Self-dispersed pigments are typically used when high density and sharp images are required such as for the printing of text and graphics, and are especially useful when printing on to plain papers (ie. papers not specifically designed to render photographic quality images).
Self-dispersed pigments useful for inkjet printing have been prepared by a number of different processes. U.S. Pat. Nos. 5,554,739; 5,803,959; and 5,922,118 disclose covalent functionalization of pigment surfaces using diazonium compounds. U.S. Pat. Nos. 5,609,671; 5,718,746; 6,099,632; and 7,232,480 describe anionic self-dispersed pigments prepared by a hypochlorite oxidation process. U.S. Pat. No. 6,852,156 describes anionic pigments prepared by ozone oxidation.
Among the different types of self-dispersed pigments, those having a high degree of surface functionalization provide advantages in the printing of inkjet images. US Patent Publication No. 2007/0028800 discloses self-dispersed pigments having a charge equivalence of at least 0.5 mEq/g that have been carboxylate functionalized. U.S. Pat. No. 5,861,447 and US Patent Publication No. 2008/0206465 disclose self-dispersed pigments having greater than 11 weight % volatile surface functional groups.
Although self-dispersed pigments have a number of advantages when used in inkjet inks, they also present disadvantages. For example, self-dispersed pigment inks are particularly susceptible to smearing, especially with respect to high-lighter markers used in the marking of text images. It known in the art of self-dispersed pigment inks to add water-soluble polymers, neutralized with organic or inorganic bases, to improve the smear resistance of the printed images. The presence of polymers in the inks can present additional limitations in ink performance. The presence of significant amounts of polymers in a self-dispersed pigment ink, e.g., can reduce the amount of achievable density in the printed image. US Patent Pub. No. 2010/0092669 discloses inkjet inks comprising water, a self-dispersing carbon black pigment having greater than 11 weight % volatile surface functional groups, and a water soluble polymer containing carboxylate groups, wherein the ink also contains an organic base having a pKa>7.5 and an optional inorganic base in combined amounts sufficient to provide alkaline equivalents of at least 150% of the acid equivalents of the water soluble polymer, where the equivalents of the organic base are greater than or equal to the equivalents of the inorganic base. Such inks are described as providing high print density and text sharpness when printed onto an ink receiving medium, and reduced polymer deposits on components of the printing system during periods of latency.
A component of nearly all modern day inkjet printers is an ink tank that delivers ink to the printhead in order to render a printed image. The ink tank prevents leakage of the ink during manufacture, storage, transportation, and the printing operation itself. The ink tank should be capable of containing the ink even under conditions where the pressure within the tank changes due to environmental conditions. For example, pressure variations within an ink tank can occur due to changes in ambient temperature such as when a tank is stored at elevated temperatures in a warehouse or a particular geographic region where high temperatures are encountered. Pressure variations within an ink tank can also occur when the tank is subjected to changes in barometric pressure such as transporting the tank in an airplane or a geographic elevation high above sea level. To this extent, most modern day inkjet ink tanks are designed with some means of pressure regulation to prevent loss of ink during substantial changes in temperature or pressure.
Various designs for regulating the pressure within an inkjet ink tank are known including, bubble generators, reverse bubblers, diaphragms, capillary media and bags. Each of these designs has limitations in the overall system performance of the tank. Ink tanks that use capillary media, such as a foam, fiber or felt, to store ink as a means for pressure regulation have the disadvantage that ink resides directly in the small passages of the capillaries. This is particularly problematic for pigmented inks since pigment particles having sizes greater than about 20 nanometers in diameter are subject to settling phenomena. This is certainly the case for most modern day pigmented inks that have particle diameters in the range of 20 to 500 nanometers.
Pigmented ink can remain in an ink tank for several years from the time of manufacture through storage and use of the tank and this provides ample opportunity for the pigment particles to settle. Ink tank designs where ink is stored in capillary media leads to a situation where pigment particles are restricted in motion within the small passages of the capillary media. This restriction in particle movement is further complicated by the so-called Boycott Effect, wherein the observed sedimentation rate is increased in proportion to the available horizontal surface area within a capillary. For a more detailed description of the Boycott Effect see, Boycott, A. E., Nature, 104: 532, 1920. Both complications lead to an inhomogeneous distribution of pigment particles within the ink carrier fluid that can manifest itself as defective images during the printing process. For example, the non-homogeneous pigmented ink can result in images having a textured appearance reminiscent of a wood grain appearance if the pigmented ink is stored in the capillary media within an ink tank. This leads to a limitation in the selection of the pigment particle size since larger particles, which can be beneficial to providing higher optical density in printed regions, are disadvantaged from a settling and homogeneity standpoint when stored in a capillary media.
A further limitation for ink tanks using capillary media is the wasted ink associated with the capillary media. Ink tank designs where capillary media is used to store ink can result in a finite amount of ink that remains trapped in the capillary media at the end of the useful life of the tank. Ink that remains trapped is effectively wasted ink as it is not available for transport to the printhead and ultimately for printing of an image. It would be desirable to minimize the amount of ink trapped in the capillary media of an ink tank.
Designs are known for ink tanks having a free ink compartment and a capillary media compartment vented to the atmosphere and in fluid communication with ink in the free ink compartment, such as U.S. Pat. Nos. 5,682,189, 5,703,633, 6,880,921, 7,252,378, and 7,290,871. In such multi-compartment ink tanks, when sufficient pressure differential exists between compartments, air from the atmosphere is provided through the vented capillary media compartment and into the free ink compartment, causing bubbles to enter the free ink compartment, and at least partially reduce the pressure differential. Such multi-compartment ink tanks may thus be described as “bubbler” ink tanks. Designs for inkjet tanks are also known where two capillary media of different porosities are present in a chamber where ink is stored, such as U.S. Pat. Nos. 5,233,369, 5,453,771, 6,186,621, 6,431,672, and PCT International Publication Number WO 2007/138624. US Pat. Pub. Nos. 2009/0309940 and 2009/0309941 disclose multi-compartment ink tanks with further desirable features.